1. Field of the Invention
The present invention relates to a polishing apparatus, and more particularly to a polishing apparatus for polishing a substrate such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Conventionally, a polishing apparatus for polishing a substrate such as a semiconductor wafer to a flat mirror finish comprises a turntable having a polishing pad or a grinding stone (fixed abrasive) thereon, and a top ring for holding the substrate. The substrate to be polished is placed between the polishing pad or the grinding stone on the turntable and the top ring. While the substrate is pressed against the polishing pad or the grinding stone under a certain pressure by the top ring, the turntable and the top ring are respectively rotated to cause a relative motion therebetween for thereby polishing the substrate.
In the polishing apparatus with the polishing pad on the turntable, a polishing liquid is supplied to a polishing area of the polishing pad. On the other hand, in the polishing apparatus with the grinding stone (fixed abrasive) on the turntable, water is supplied to a polishing area of the grinding stone. A grinding stone impregnated with a lubricating liquid can dispense with the liquid supply from an external source. The substrate is polished to a flat mirror finish in a polishing process performed by the polishing apparatus thus constructed. After the substrate is polished, the substrate is released from the top ring and delivered to a next process such as a cleaning process.
While the substrate is polished by the polishing apparatus, the substrate held by the top ring is pressed against a polishing surface of the polishing pad or the grinding stone. As a result, polishing performance of the polishing pad or the grinding stone may be deteriorated due to glazing of the polishing surface, or the polishing surface of the polishing pad or the grinding stone may have undulation beyond an allowable degree. In such cases, the polishing pad or the grinding stone is dressed to recover a desired polishing surface.
In this type of conventional polishing apparatus, it has heretofore been necessary to manage the timing of dressing of the polishing pad or the grinding stone based on the number of polished substrates or the polishing time, e.g., the time when the polishing pad or the grinding stone has been used to polish substrates. Further, in order to recognize how the polishing surface has changed, it is necessary to remove the polishing pad from the turntable, and then remove a soft layer of the polishing pad and measure configurational changes, e.g., changes in thickness of a hard layer of the polishing pad.
The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a polishing apparatus which can easily measure changes in condition of a polishing surface, can appropriately determine when to dress the polishing surface and to replace a component of the polishing surface, and can polish a surface of a substrate to a high-quality finish.
According to an aspect of the present invention, there is provided a polishing apparatus for polishing a workpiece, the polishing apparatus comprising: a polishing table having a polishing surface; a top ring for holding the workpiece and pressing the workpiece against the polishing surface; a dresser for dressing the polishing surface; and a sensor for observing a property of the polishing surface on the polishing table when the polishing surface is being dressed by the dresser. The polishing apparatus may further comprise a display device for displaying the property of the polishing surface observed by the sensor.
When the polishing surface is dressed by the dresser, a property of the polishing surface on the polishing table can be observed by the sensor. For example, the property of the polishing surface may be irregularity or undulation thereof. The observed property may be displayed by the display device. Therefore, the property of the polishing surface can easily be recognized.
The sensor may be mounted on a fixed member of the top ring or the dresser which is angularly movable. The sensor may comprise a displacement sensor.
In a preferred aspect of the present invention, the sensor is mounted on one of the top ring and the dresser and is vertically movable independently of the top ring or the dresser.
Since the sensor is mounted on the top ring or the dresser and is vertically movable independently of the top ring or the dresser, the position of the sensor can easily be adjusted with respect to the polishing surface. It is thus easy to adjust the sensor to a position optimum for measuring the property of the polishing surface.
At least a portion of the sensor which is brought into contact with a polishing liquid or a dressing liquid may be made of a material having chemical resistance. Hence, the sensor is not corroded by the polishing liquid or the dressing liquid, and durability of the sensor can be improved.
In a preferred aspect of the present invention, the sensor measures a property of the polishing surface over an area larger than an area which is dressed by the dresser.
Since the sensor measures a property of the polishing surface over an area larger than an area which is dressed by the dresser, the property of the polishing surface can reliably be measured.
In a preferred aspect of the present invention, the polishing apparatus further comprises a determination device for comparing an initial property of the polishing surface which is measured by the sensor with a property of the polishing surface which is measured by the sensor after the polishing surface is dressed by the dresser, and determining when to replace a component of the polishing surface based on the result of comparison.
Since the polishing apparatus comprises the determination device, the initial property of the polishing surface can be compared with the property of the polishing surface after the polishing surface is recovered by the dresser. Therefore, a component of the polishing surface can be replaced with a new one at an optimum time.
According to another aspect of the present invention, there is provided a method for polishing a workpiece, comprising: holding the workpiece and pressing the workpiece against a polishing surface on a polishing table to polish the workpiece; observing a property of the polishing surface on the polishing table by a sensor when the polishing surface is being dressed by a dresser; comparing an initial property of the polishing surface which is measured by the sensor with a property of the polishing surface which is measured by the sensor after the polishing surface is dressed by the dresser; and determining when to stop the dressing operation based on the result of comparison.
Since an initial property of the polishing surface is compared with a property of the polishing surface after the polishing surface is dressed by the dresser, it is easy to determine when to stop the dressing operation based on the result of comparison. Therefore, the dressing operation can be stopped at an optimum time.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.